This invention relates to a process for applying a microcapsule-containing coating composition to paper. The process is particularly useful for applying microcapsule coatings as used in pressure-sensitive copying paper, or carbonless copying paper as it is more usually referred to.
Carbonless copying paper sets typically comprise an upper sheet coated on its lower surface with microcapsules containing a solution in an oil solvent of at least one chromogenic material (alternatively termed a colour former) and a lower sheet coated on its upper surface with a colour developer composition. If more than one copy is required, one or more intermediate sheets are provided, each of which is coated on its lower surface with microcapsules and on its upper surface with colour developer composition. Imaging pressure exerted on the sheets by writing, typing or impact printing (e.g. dot matrix or daisy-wheel printing) ruptures the microcapsules thereby releasing or transferring chromogenic material solution on to the colour developer composition and giving rise to a chemical reaction which develops the colour of the chromogenic material and so produces a copy image.
In an alternative type of carbonless copying paper, the microcapsules and the colour developer are applied to the same surface of the paper, either in a single layer or in two separate layers.
Various techniques have been used for applying the microcapsule coatings required in carbonless copying papers. The technique used originally involved applying an excess of an aqueous microcapsule coating composition to the paper by means of an applicator roll, and then metering the wet coating to the desired coatweight by means of an air knife. The paper web was guided so as to kiss or contact the upper part of the applicator roll, with the lower part of the roll dipping into a bath of coating composition. The applicator roll was continuously rotated such that its surface in contact with the web moved in the same direction as the moving web (forward-roll coating). Such an arrangement is disclosed, for example, in British Patent No. 974497.
A modified form of roll/air knife coating was later introduced, and is disclosed for example in British Patent No. 1151690. In this arrangement, a rotating pick-up roll dips into a bath of coating composition and is arranged to transfer the picked up coating to an applicator roll running in contact with the paper web. A metering roll positioned at a precise spacing from the applicator roll is provided to meter off excess coating composition transferred from the pick-up roll. The spacing of the metering roll from the applicator roll is termed the metering gap, and the width of this gap is the primary determinant of the thickness, and hence the wet coatweight, of the applied coating. Fine adjustment of wet coatweight can be achieved by adjustment of the applicator roll speed relative to the web speed (adjustment of the metering roll speed to suit the applicator roll speed may also be necessary). As disclosed in British Patent No. 1151690, the pick-up roll may rotate in either the same or the opposite sense as the applicator roll. The metering roll always rotates in the same sense as the applicator roll (so that their adjacent surfaces at the metering gap move in opposite directions). The web runs counter to the direction of movement of the applicator roll surface at the point of contact of the web and the applicator roll (reverse-roll coating). An air-knife is provided for final metering to the desired coatweight.
Gravure coating (also termed "flexographic" coating) has also been widely used for applying microcapsule coatings, particularly for "on machine" coating, i.e. coating the web immediately after it has been produced on the papermachine, with no intermediate reel-up and transport to a separate coating machine. Such a technique is disclosed, for example, in British Patent No. 1253721. A further proposal for gravure application of microcapsule coatings is to be found in European Patent Application No. 37682 A.
Gravure coating is particularly suited to the application of coatings at a low wet coatweight. This means that gravure coating can only be successfully used in the production of carbonless copying papers when high solids content microcapsule coating compositions are to be applied. By "high solids" in this context is meant microcapsule coating compositions of a solids content of the order of around 40% or more, and of which the microcapsules have synthetic polymer walls rather than the more traditional gelatin coacervate walls. Not all manufacturers of pressure-sensitive copying papers are able or wish to use such high solids microcapsule coating compositions. Gravure coating also has other drawbacks which for some manufacturers outweigh its advantages, and in any case the cost of converting from non-gravure coating to gravure coating can be high.
A further microcapsule coating process which is said to be in commercial use relies on the use of a Dahlgren LAS coater. This utilises a resilient roll which dips into a bath of coating composition and also runs in nip pressure contact with a hard steel applicator roll. The resilient roll and the applicator roll rotate in opposite senses so that their surfaces run in the same direction at the nip between them. The resilient roll serves both to pick up coating composition from the bath and to meter a desired amount of the coating on to the surface of the applicator roll. The applicator roll also runs in nip pressure contact with a resilient backing roll, with the paper web running between the applicator roll and the backing roll in a direction counter to the direction of movement of the surface of the applicator roll with which it is in contact, i.e. in a reverse-roll coating mode. This means that the film split pattern produced at the metering nip between the resilient roll and the applicator roll should not be a major problem, as reverse roll coating should smooth out such a pattern.
Thus at the present time, there is no universally employed technique for applying microcapsule coatings in the production of carbonless copying paper. Non-gravure roll coating techniques based on those disclosed in British Patent No. 1151690 remain in widespread use. A number of modifications have however been made or proposed in relation to the process and apparatus disclosed in British Patent No. 1151690. For example, advances in metering roll technology have made it possible to meter very precisely the coatweight applied to the paper by the applicator roll, and thereby to dispense with the need for secondary metering by means of an air knife.
British Patent No. 1460201 proposes feeding the microcapsule coating composition direct to the metering nip of a coater working on the principles disclosed in British Patent No. 1151690. This dispenses with the need for a separate pick-up roll. British Patent No. 1460201 also discloses that the applicator roll may if desired be rotated in a sense such that its surface moves in the same direction as the web at the point of contact of the web and the applicator roll, rather than running counter to the web as disclosed in British Patent No. 1151690. This constitutes a change from reverse roll coating to forward roll coating. A three-roll coating head for forward roll application of microcapsule coatings is also disclosed in FIG. 7 of British Patent No. 1433165.
Forward roll coating has the advantage that it presents less problems of web tension control and runnability at high coating speeds than does reverse roll coating. On the other hand, forward roll coating has the drawback that film splitting occurs as the web parts company with the applicator roll, with the result that the wet coating on the web exhibits an uneven film-split pattern. This problem can be countered by the provision of reverse-turning smoothing rolls positioned downstream of the coating head. Such rolls are known in themselves, and are disclosed, for example, in British Patent No. 974497 referred to above (this patent also discloses a forward roll coating process which gives rise to a film-split pattern). The action of the smoothing rolls is to redistribute the wet coating on the web and so erase the film-split pattern. The smoothing rolls do not have a metering action, i.e. they do not remove coating composition from the web. Although beneficial in terms of producing an improved coating pattern, the use of smoothing rolls is disadvantageous in that it makes control of the web tension both more difficult and more critical than if no smoothing rolls are employed.
Whilst microcapsule-coating techniques based on the metering roll coating process disclosed in British Patent No. 1151690 have proved themselves over the years, metering gap techniques are inherently limited in relation to the minimum wet coatweight which may be applied. This is because the wet coatweight is determined primarily by the width of the metering gap, as explained earlier. The width of this gap varies slightly as the rolls rotate, owing to inevitable imperfections in the roll bearings, and in the "roundness" of the rolls. Thermal expansion of the rolls can also affect the width of the metering gap. In most cases, variations arising for the reasons just mentioned are insignificant in relation to the width of the gap, but as the coatweight diminishes, this ceases to be so. Thus attempts to apply very low coatweights using metering gap technology are likely to result in a coating of uneven thickness. There is also a risk that the metering and applicator rolls could touch. Since these rolls are conventionally of steel, contact of the rolls at high speeds would almost certainly result in serious damage.
In the past, the low coatweight limitation of metering gap coating has not been a problem in the case of microcapsule coatings, since the wet coatweights needed have been above the wet coatweight threshold at which problems of the kind outlined above become significant. However, advances in microencapsulation technology are making it possible to obtain higher solids content microcapsule coating compositions, not only in the case of microcapsules having synthetic polymer walls, but also in the case of gelatin-based microcapsules. These higher solids content microcapsule compositions require the application of a lower wet coatweight to achieve the same dry coatweight and are advantageous in two respects. Firstly, less water has to be evaporated off in the drying stage, which saves energy. Secondly, a better sheet appearance results since the paper is not wetted to the same extent (less wetting of the paper reduces the tendency of the finished paper to curl and to cockle).
Metering gap coating processes appear to be inherently unlikely to be capable of meeting the likely long term future needs for the application of high solids content microcapsule coating compositions, because of the low wet coatweight limitations discussed above. But quite apart from the limitations associated with the metering gap itself, currently known metering gap coating technology has other limitations when considered in relation to higher solids content microcapule coating compositions. Firstly, the higher viscosity of such compositions inhibits proper transfer of the microcapsule coating from the applicator roll to the web as the web passes over the applicator roll. Secondly, the wet coatweights applied when higher solids coating compositions are used are so low that reverse-turning smoothing rolls would not be fully effective to smooth out the film split pattern inevitably produced with forward roll coating. This could not simply be remedied by operating in a reverse-roll mode, as reverse roll coating is unsuited to very high coating speeds. This is because it becomes very difficult to control the web tension properly, which leads to inconsistent coating and web breakages.
A further factor is that for a given wet coatweight, reverse roll coating generally requires a smaller metering gap than does forward-roll coating. This is because in reverse roll coating, the applicator roll speed has to be equal to or greater than web speed in order to give a uniform distribution of coating composition, whereas for forward roll coating, the applicator roll runs at a fraction of the web speed. The speed of the applicator roll relative to the web speed affects the coatweight applied, and therefore the faster running applicator roll used in reverse roll coating will apply a higher coatweight at a given web speed and metering gap. Thus in order to obtain a particular coatweight, a lower metering roll gap is needed in the case of reverse roll coating. The inherent metering gap limitations therefore bear more harshly on reverse-roll coating than on forward roll coating.
It is an object of the present invention to overcome or at least minimise the problems described above and to provide an improved high speed forward roll coating process for applying microcapsule-containing coating compositions to paper. The present invention also seeks to provide a process which can be taken up at a relatively low conversion cost by a paper mill which currently uses non-gravure roll coating for applying microcapsule coating compositions and which wishes to avoid the risk of switching to a fundamentally different type of coating process, for example a gravure coating process or the Dahlgren process, of which it has no experience.
The present invention achieves the above objectives by dispensing with metering gap metering and instead controlling coatweight by means of a metering roll which is deformable rather than hard and which rotates in pressure contact with the applicator roll. A deformable smoothing roll is also provided to run in contact with the applicator roll to smooth the metered coating, and a soft backing roll is provided at the point of contact of the applicator roll and the web so as to afford good transfer of the coating from the applicator roll to the web without significant film splitting. This dispenses with the need for smoothing rolls positioned downstream of the coating head.
The use of a rubber-covered smoothing roll in contact with a steel applicator roll was in fact first proposed over 40 years ago in U.S. Pat. No. 2398844. This patent issued on 23rd Apr. 1946 to Gerald D. Muggleton and Albert F. Piepenberg, and was assigned to Combined Locks Paper Co. The coater forming the subject of this patent became well known as the Combined Locks coater, and is referred to in a number of standard reference books, for example "Coating Equipment & Processes" by George L. Booth; Tappi Monograph No. 28 entitled "Pigment Coating Processes"; and "Pulp and Paper", by James P. Casey. The Combined Locks pigment coater design has thereby been given wide exposure. Despite this, it has not previously been appreciated that the problems described above in relation to the application of microcapsule containing coating compositions can be avoided by a process which, inter alia, utilises a deformable smoothing roll running in contact with a hard applicator roll.
According to the invention, there is provided a process for applying a microcapsule-containing coating composition to paper, comprising the steps of
feeding coating composition to a region of contact between a hard applicator roll and a deformable metering roll which rotate in opposite senses such that their surfaces at the region of contact move in the same direction and define an ingoing nip;
maintaining gentle pressure between the applicator and metering rolls and controlling their relative speeds so as to permit only a controlled amount of coating composition to pass through said nip and to leave a metered amount of coating composition on the surface of the applicator roll after it has left said region of contact;
smoothing the metered amount of coating composition remaining on the surface of the applicator roll by means of a deformable smoothing roll which rotates in the same sense as the applicator roll and in contact therewith; and
transferring the smoothed coating composition on the surface of the applicator roll to a paper web which runs in the same direction as, and no slower than, the surface of the applicator roll carrying the smoothed coating composition and which is held in temporary contact with the applicator roll by a soft backing roll which rotates in an opposite sense to the applicator roll so as to form an ingoing nip therewith.
The applicator roll surface preferably runs at least about 75 to 80% of the web speed, and may approach web speed. The optimum ratio between the applicator roll speed and the web speed may vary somewhat, depending on the web speed. By way of example, an applicator roll surface speed of about 990 to 995 m min.sup.-1 (i.e. 99 to 99.5% of web speed) has been found to be advantageous for a web running at about 1000 m min.sup.-1. The optimum relative web and applicator roll surface speeds will also depend on other factors as well, particularly the viscosity of the microcapsule composition being applied.
Although the present invention is particularly suited to the application of high solids content high viscosity microcapsule compositions, it may of course also be used for the application of lower solids content lower viscosity microcapsule compositions.
In order to enable the invention to be more readily understood, reference will now be made to the accompanying drawings which depict diagrammatically and by way of example an embodiment thereof and data relevant thereto, and in which: